Quantifying Filopodia in Cultured Astrocytes by an Algorithm.

Astrocytes in vivo extend thin processes termed peripheral astrocyte processes (PAPs), in particular around synapses where they can mediate glia-neuronal communication. The relation of PAPs to synapses is not based on coincidence, but it is not clear which stimuli and mechanisms lead to their formation and are active during process extension/ retraction in response to neuronal activity. Also, the molecular basis of the extremely fine PAP morphology (often 50 to 100 nm) is not understood. These open questions can be best investigated under in vitro conditions studying glial filopodia. We have previously analyzed filopodial mechanisms (Lavialle et al. PNAS 108:12915) applying an automated method for filopodia morphometry, which is now described in greater detail. The Filopodia Specific Shape Factor (FSSF) developed integrates number and length of filopodia. It quantifies filopodia independent of overall astrocytic shape or size, which can be intricate in itself. The algorithm supplied here permits automated image processing and measurements using ImageJ. Cells have to be sampled in higher numbers to obtain significant results. We validate the FSSF, and characterize the systematic influence of thresholding and camera pixel grid on measurements. We provide exemplary results of substance-induced filopodia dynamics (glutamate, mGluR agonists, EGF), and show that filopodia formation is highly sensitive to medium pH (CO2) and duration of cell culture. Although the FSSF was developed to study astrocyte filopodia with focus on the perisynaptic glial sheath, we expect that this parameter can also be applied to neuronal growth cones, non-neural cell types, or cell lines.

Analysis of proton-density bias corrections based on T1 measurement for robust quantification of water content in the brain at 3 Tesla.

PURPOSE: Estimating tissue water content using high field MRI, such as 3 Tesla (T), is challenging due to the difficulty in dissociating the radio frequency inhomogeneity pattern from the signal arising from tissue intrinsic proton density (PD) variations. To overcome this problem the longitudinal relaxation time T1 can be combined with an initial guess of the PD to yield the desired PD bias correction. However, it is necessary to know whether T1 effects, i.e., any effect contributing to T1 while being independent of tissue hydration, influence the estimated correction. METHODS: Twenty-five healthy subjects underwent a quantitative 3T MRI protocol enabling acquisition of 64 slices with 1 mm in-plane resolution and 2 mm slice thickness in 14 min. Influence of T1 effects on the estimated water content map is evaluated using a dedicated method including T1 and T2 * information and region of interest-based water content values are compared with the literature. RESULTS: Our analysis indicates that the PD bias correction based on T1 is largely insensitive to T1 effects. Besides, water content results are in good agreement with literature values obtained at 1.5T. CONCLUSION: This study demonstrates the applicability of a PD bias correction based on T1 to yield tissue water content at 3T.

MR parameter quantification with magnetization-prepared double echo steady-state (MP-DESS).

PURPOSE: The mapping of MR relaxation times and proton density has been the subject of research in medical imaging for many years, as it offers the possibility for longitudinal investigation of disease and the correlation with related biochemical processes. The purpose of this study is to provide a fast mapping protocol, which simultaneously acquires MR relaxation times and relative proton density without compromising accuracy and precision. METHODS: This work presents a novel magnetization-prepared double echo steady-state (MP-DESS) sequence, which was designed to be sensitive to parameter variations of interest, and insensitive to variations of confounding variables. It provides high sensitivity against variations of the MR relaxation times, high acquisition efficiency, and it is insensitive to off-resonance. Accurate phase graph modeling of the MP-DESS signal is used to obtain unbiased parameter estimates. RESULTS: The approach is validated in phantom and in vivo measurements. A whole-brain acquisition of 1.4-mm isotropic resolution was acquired in 15 min. Comparisons to gold-standard methods suggest a mapping precision of 5% for T1 and M0 , and below 10% for T2. CONCLUSION: A new quantitative imaging technique is introduced that allows fast and isotropic simultaneous MR parameter mapping of T1, T2, and M0.

Investigation of the spatial correlation in human white matter and the influence of age using 3-dimensional variography applied to MP-RAGE data.

A novel method for the quantification of heterogeneity and spatial correlation in 3D MP-RAGE images of white matter is presented. The technique is based on the variogram, a tool commonly used in geosciences for the analysis of spatial data, and was tailored to the special requirements of MR image analysis. Influences from intensity non-uniformities, noise and arbitrary greyscale were quantified and considered in the calculations. The obtained variograms were fitted with spherical model functions to infer parameters that quantify heterogeneity and size of the correlation structures of the tissue. Numerically generated samples with well-defined correlation properties were employed to validate the estimation process and to provide an interpretation of the parameters obtained. It is shown that the method gives reliable results in an interval of correlation structures sized between 2mm and 20mm. The method was applied to 24 MP-RAGE datasets of healthy female volunteers ranging in age from 19 to 73 years. White matter was found to have two prominent correlation structures with sizes of approximately 3mm and 23 mm. The heterogeneity of the smaller structure increases significantly with age (r=0.83, p<10(-6)).